Investigate is_core_test() operation & logic during certification

Issue Summary

The is_core_test() was written and tested for core/contrib testing, but certification was not part of the test plan. Now, we see invalid key errors in watchdog during certification testing. This is already patched in 6.x-2.x, but we need to take a closer look and see how this code behave in the case of a certification test; as this wasn't considered in writing the patch.